Rashi: He promised me that my greatness will increase, like the morning light gets stronger.

Targum Yonasan: Tzadikim in the world to come will illuminate like the morning, and like the sun in the future, which will be far stronger than now.

Malbim: Kingship of this world, including of David, is like the gradual increase of light from dawn (which is barely light) until sunrise. The kingship of Mashi'ach in the future will be different. Rather, at the time of dawn, immediately a great light will shine. The prefix Chaf in uch'Or does not mean 'like', rather, at the time of. Mashi'ach will immediately rule over the world.

Rashi: Light that is not dark.

Radak: A morning that is not cloudy.

Malbim: Kingship of this world is compared to the sun shining. Sometimes it is cloudy, and the sun is not seen at all. So happened to David's kingship when he was exiled. The future kingship of Mashi'ach will not be cloudy.

Rashi: He illuminates my morning more than light that comes from rain on grass of the land. "K'Chom Tzach Alei Or." When rain falls on land full of grass and the sun shines on it, it gleams.

Targum Yonasan: [Tzadikim will rejoice in the world to come like a farmer who was longing] for rain for his crops [in years of famine, and the years of rain came].

Radak: [A morning that is cloudy] sometimes there is light, and sometimes rain. It is good only to make grass grow.

Malbim: Sometimes the sun is seen through falling rain. Rain is needed to make the grass sprout, but one does not see the sun clearly through the rain, rather, a mere Nogah (something that receives light). Sometimes when conducting kingdoms via nature, it is needed to have kingdoms parallel ¹ ; they dim its light. This will not apply to the future kingship of Mashi'ach.